The present invention generally relates to compressors and expanders, and more particularly, to a unitary rotary sliding vane type compressor and expander for use in conjunction with a Proton Exchange Membrane (PEM) fuel cell.
The present invention, although not limited to any particular application, arose from a lack in the prior art of a satisfactory compressor and expander for use in conjunction with a modem, very small, highly efficient fuel cell. Fuel cells generate electricity as a result of electrochemical interactions that occur inside the fuel cell between a fuel such as hydrogen and an oxidant such as air. Such fuel cells have an anode space and a cathode space which are separated from one another by a proton exchange membrane. Electricity is generated when oxidant is introduced to the cathode space and fuel is introduced to the anode space. Hydrogen fueled fuel cells are disclosed, for example, in U.S. Pat. Nos. 5,645,950, 4,657,829, and also in U.S. Pat. No. 6,124,051 (hereinafter the 051 patent), assigned to the assignee of the present invention. The ""051 patent is incorporated herein in relevant part by reference.
It is known in certain industrial and in the automotive art to improve the operating efficiency of fuel cells by pre-compressing the oxidant gas entering the fuel cell, while expanding the spent oxidant gas exhausted from the fuel cell. Such prior art systems have typically utilized high-speed impellers, or other turbomachinery or turbocharger-like compressors and turbines for this purpose. Systems for pre-compressing and then expanding the fuel cell oxidant are disclosed, for example, in U.S. Pat. Nos. 4,657,829, 5,645,950, 5,981,096, among others. However, fuel cells of ever smaller size are being developed for applications requiring much lighter weight, much more compact, and more efficient electrical generation systems. Illustrative of a small, efficient fuel cell suitable for such applications is the fuel cell disclosed in the ""051 patent. In such smaller systems it becomes necessary to also scale down the size of the prior art compressors and turbines to satisfy size and weight constraints imposed by the system requirements. The inventors of the present invention have discovered that as a consequence, it would be necessary to operate scaled down prior art compressors and turbines at excessively high rotational speeds to provide an adequate volume flow, resulting in dynamic unbalance problems. In addition, the efficiency of prior art turbomachine type compressors and turbines drops off dramatically below a certain volume flow rate. The inventors further discerned and discovered that dynamic unbalance and loss of efficiency could preclude the practical application of the prior art compressors and turbines in small and lightweight electrical generation systems.
It is also known in the prior art to improve the operating efficiency of a fuel cell by pre-humidifying the oxidant gas flow entering the fuel cell. For example, in U.S. Pat. No. 5,645,950 is described a system in which product water that is contained in the process air after it has passed through the fuel cell is separated by one or more liquid separators from an air discharge line, and collected in a storage container. The water required for humidifying is then drawn from the container and injected into the fuel cell air supply line. Although perhaps suitable for prior art applications, such separating and humidifying devices are prohibitively heavy and complex for use in conjunction with a small fuel cell of the type previously described.
Accordingly a need exists for a suitable compact, simple, and lightweight compressor and expander for use in conjunction with a fuel cell. Another need exists for a suitable small size and lightweight device for pre-humidifying the oxidant gas flow to a fuel cell.
In one embodiment of the invention, a unitary compressor-expander is provided comprising a housing having a compressor side with a compressor intake and a compressor outlet, and a turbine side with a turbine intake and a turbine exhaust. A cylindrical rotor is disposed within the housing with a plurality sliding vanes disposed in slots around an outer periphery thereof, wherein the vanes are configured to slide outwardly along the slots upon rotation of the rotor, and sealingly contact an inner contoured surface of the housing. The bottom of a vane slot may be vented through a passage in a cover plate to at least one neighboring vane slot bottom, and to the compressor intake manifold. Alternatively the vane slot bottoms may be vented by a groove between the vane and vane slot to the compression or exhaust chambers.
In another embodiment of the invention a magnet is inserted in at least one of the vanes, and at least one stationary magnet is disposed in the housing about the inner contacting surface. The poles of the stationary magnets may be preferentially oriented so as to attract or repel the vane mounted magnets, thereby increasing or decreasing the contact force between the vane tip and housing. A ferrous metal insert may be used in the vanes instead of a magnet, in which case the stationary magnets can be used to increase the force of the blade tip against the housing where desired.
In another embodiment of the invention an integral condenser-humidifier is disposed in the path of the expanded gas exhausting from the turbine outlet, for condensing water out of the expanded gas and returning the condensed water to the compressor-expander. The integral condenser may comprise a substantially vertically oriented spout or an internal chamber positioned to allow condensed water to drain back into the turbine exhaust manifold and into the path of the vanes. The vanes carry the water over to the compressor portion of the compressor-expander, thereby humidifying the compressed air and improving the sealing of the contacting surfaces therein.
In yet another embodiment of the invention an electrical generation system is provided comprising a unitary sliding vane type compressor-expander and a fuel cell. The compressor portion of the compressor-expander provides compressed air to the oxidant inlet of the fuel cell, and the spent oxidant gas exhausted from the fuel cell is expanded across the expander portion of the unitary compressor-expander. An integral condenser-humidifier may be provided in the path of the expanded gas exhausting from the expander outlet for condensing water out of the expanded gas and returning the condensed water to the compressor-expander. The condensed water lubricates and seals the unitary vane compressor-expander and humidifies the fuel cell, improving the efficiency of both.